btrfs: move root helpers back into ctree.h

These accidentally got brought into accessors.h, but belong with the
btrfs_root definitions which are currently in ctree.h.  Move these to
make it easier to sync accessors.[ch] into btrfs-progs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>